The alloreactive T cell repertoire has been analyzed for responses to two categories of alloantigens: mutant Kb determinants and non MHC-encoded Mls antigens. It was demonstrated by limiting dilution techniques and slope analysis that proliferating F1 T cell populations contain distinct subsets capable of recognizing Mlsc encoded determinants in the context of parental MHC products. These findings demonstrate that Mlsc determinants are recognized by responding T cells in association with MHC encoded determinants. T cell clones specific for Mlsa were recently generated, and the MHC restriction of recognition by these clones was evaluated. The specificity of these clones for Mlsa products was first established employing a variety of inbred strains including recombinant inbred lines. Studies of H-2 congenic strains demonstrated that cloned T cells recognize Mlsa in the context of H-2 determinants expressed by some but not all haplotypes. It was also observed that a high proportion of clones specific for soluble antigens (antigen-specific), foreign T region products (alloreactive), or syngeneic I products (autoreactive) showed cross-reactive recognition of Mlsa in an H-2 restricted fashion. Thus H-2 restricted recognition of Mlsa products occurs in high frequency in the T cell repertoire. Cloned T cells specific for Mlsa were capable of mediating skin graft rejection in nude (T-deficient) recipients, suggesting that Mls determinants may serve as targets for graft rejection in vivo. Responses to Kb mutant determinants were also evaluated employing radiation bone marrow chimeras, neonatal tolerization, and cold target inhibition in assays of cell mediated lympholysis (CML). The results of such studies demonstrated that the generation of the T cell repertoire to these mutant MHC determinants was not the result of T cell genetype alone or of muturation environment alone, but rather represented the outcome of unique interactions between these two variables. This T cell repertoire appears to reflect, at least in part, the activation requirements of cytotocic T cell precursors, and to be regulated by genes in the K region of the MHC.